WHEN I WAS a kid, I had a rock collection, including a box of polished thundereggs and other colorful stones. At some point I got rid of them. Now I’m collecting rocks again, except these rocks are expensive, useful — and painful.

These are not semiprecious stones. They’re Japanese waterstones for sharpening knives. “Japanese waterstone” has a romantic ring to it, but these stones don’t come from ancient quarries in Hokkaido; they’re made in factories from synthetic materials. (They require the use of water when sharpening; hence the name.)

This obsession started last year when I bought my first Japanese knife and joined the ranks of cooks who know that the sharpest and most elegant knives in the world are made in Japan. The problem is, once you use a truly sharp knife, once you slice a potato with mandoline-like precision, you never want to use anything else.

Keeping a knife that sharp (“scary sharp,” to knife buffs) requires frequent touch-ups, and the sharpening tools made for German knives — electric sharpeners, honing steels — don’t work well on Japanese knives.

One of America’s best knife sharpeners, Daniel O’Malley, runs the Epicurean Edge in Kirkland. His skills are unimpeachable, but he charges about $20 to sharpen a chef’s knife, and how often did I want to go over to Kirkland with Andrew Jackson in my hand?

“Should I try sharpening my own knives?” I asked O’Malley.

“Well, I sell a lot of sharpening stones,” he said. “But I don’t know how many actually get used. It takes 40 or 50 knives before you get any good at it.”

When someone says this sort of thing to me, I am determined to prove them wrong. It didn’t help that I started reading an online forum for knife enthusiasts (www.foodieforums.com), where the prevailing opinion is that letting someone else sharpen your knives is the equivalent of letting someone else father your children.

So I began amassing a set of Japanese waterstones. Some of them are really quite lovely. I have my eye on a powder blue Naniwa polishing stone, which had better be adorable, because it costs $75. To get some practice, I sent out an e-mail to some friends offering to sharpen their knives for free, with the caveat that I could not promise that the knives would actually end up any sharper. Mary took me up on the offer. She brought three knives and a couple of Midnight Cuban sandwiches from Paseo, probably the best sandwich in Seattle. As I sharpened her knives and ate the spicy, messy pork sandwich, it occurred to me that I was now doing manual labor in exchange for food.

Then I hit a plateau. My knives were just sharp, not scary sharp. I needed professional help, so I called on Brandon Wicks. Brandon is the sushi chef/butcher at ART restaurant at the Four Seasons Hotel. His knife skills are insane, and he has a gorgeous collection of Japanese knives. He invited me to his condo in the Chinatown International District for a sharpening lesson.

I showed Brandon my technique, and he told me that I was doing everything wrong: holding the knife the wrong way, sharpening at too steep an angle, and holding the angle inconsistently. I flipped the knife around so the edge was facing me and placed two fingers near the edge to hold it against the stone. “Isn’t this dangerous?” I asked. Brandon didn’t answer. It took me about 30 seconds to cut my finger. It was a tiny cut, but by the time I dragged it over a waterstone for half an hour, it had turned into a large abrasion, and I bled all over Brandon’s kitchen.

“That’s a nice slurry there: blood, metal and stone,” he said. It was kind of like the Middle Ages in miniature. “Don’t worry, I cut myself all the time,” he added, and then he did. “We’re blood brothers now.”

When I finished sharpening, we attacked an onion and some radishes. I had done it: The knife was brutally sharp. Plus, I had donated blood.

A few days later, after my wound healed, I pulled out my stones and a dull knife and went to work. The result was the same: The knife was ready to demolish a fridge full of produce, and I cut my finger. I put on a bandage (Nexcare Waterproof, the choice of clumsy cooks), and posted to the knife forum asking for advice on how not to injure myself while sharpening. This felt like going to a cycling forum and saying, “I keep falling off my bike. Can you help?”

The problem, it turned out, was that I wasn’t keeping my fingers and the knife in sync. When I dragged the knife backward, my fingers hesitated just long enough for the edge to bite into them. It took about three more bloody incidents for my brain to accept this, and now I can sharpen without pain.

Recently, some friends asked me for a sharpening lesson. I was nervous about teaching a skill I’d only recently (and only barely) acquired myself. (O’Malley was right: I had to sharpen 40 or 50 knives before I was any good.) But it turned out fine. The knives ended up sharp, and only one guest got cut. “Don’t worry, I cut myself all the time,” I said.

So, to recap: Japanese knife-sharpening requires expensive equipment, causes injuries and takes months to learn. It’s certainly not for everyone. But isn’t that also true of other local pursuits like skiing or mountain biking?

Besides, knife-sharpening is useful. It’s not just an excuse to play with pretty rocks . . . At least that’s what I keep telling my 9-year-old self.

Matthew Amster-Burton is a Seattle freelance writer and author of “hungry monkey: A Food-Loving Father’s Quest to Raise an Adventurous Eater.” He can be reached at mamster@gmail.com. Ken Lambert is a Seattle Times staff photographer.